Quantitative analysis of single-molecule force spectroscopy on folded chromatin fibers.
Bottom Line: Comparison of force-extension curves between single nucleosomes and chromatin fibers shows that embedding nucleosomes in a fiber stabilizes the nucleosome by 10 kBT.Chromatin fibers with 20- and 50-bp linker DNA follow a different unfolding pathway.These results have implications for accessibility of DNA in fully folded and partially unwrapped chromatin fibers and are vital for understanding force unfolding experiments on nucleosome arrays.
Affiliation: Biological and Soft Matter Physics, Huygens-Kamerlingh Onnes Laboratory, Leiden University, Leiden, The Netherlands.Show MeSH
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Mentions: The discrete steps in extension at forces above 6 pN represent the sequential unwrapping of the last DNA from each nucleosome and have been studied abundantly with optical tweezers (4,27) and MTs (51). MTs act as a force clamp rather than a position clamp, resulting in a staircase-like force-extension curve instead of the typical saw-tooth pattern obtained with optical tweezers. Figure 3A shows a zoom in on these high-force transitions. The corresponding step size distribution is shown in Figure 3B. A step size of 22 ± 3 nm was found, in range with previous studies on various DNA substrates and under different buffer conditions.
Affiliation: Biological and Soft Matter Physics, Huygens-Kamerlingh Onnes Laboratory, Leiden University, Leiden, The Netherlands.